The present invention relates generally to centrifugal contactors in a multistage or cascade system for separating liquids of different weight phases, and more particularly to the modification of the centrifugal contactors functioning as the end stages in the cascade for enabling the cascade to continue operation when an end stage becomes inoperative. This invention was made as a result of work under contract DE-AC05-84OR21400 between Martin Marietta Energy Systems, Inc., and the U.S. Department of Energy.
Centrifugal contactors operating in a cascade or multistage arrangement have been found to provide a highly satisfactory system for separating liquids from one another based on different weight phases. The use of centrifugal contactors is particularly advantageous in operations where short residence times, small inventories and high separating force fields are beneficial. Typical employment of centrifugal contactors where success has been demonstrated is in areas including pharmaceuticals, processing of lubricating oils, treatment of liquid wastes and nuclear reactor fuel reprocessing. Centrifugal contactors are particularly desirable for the reprocessing of nuclear reactor fuels since in such instances short residences times, small inventories and high separating force fields are of high priority due to the high radiation fields in the process liquids as generated by fission products therein which damage the extraction solvents over extended periods of contact. Also, with small inventories criticality concerns with respect to the fissionable material in the process liquids are minimized and emulsification problems are significantly controlled by the large separating force applied to the process liquids in the centrifugal contactor.
In nuclear reactor fuel reprocessing operations, the centrifugal contactors can be utilized to separate uranium and plutonium values from fission products and other actinides undesirable for use nuclear reactor fuels. Typically, in a nuclear fuel reprocessing operation using centrifugal contactors, a cascade of several, i.e., about four to twenty four, centrifugal contactors are employed with each centrifugal contactor comprising a housing containing a vertically oriented rotor. The process liquids comprise a heavy solvent or organic phase and a lighter aqueous phase which are introduced into the centrifugal contactors through separate conduits or inlets which are in registry with a mixing zone before the liquid mixture enters the rotor where centrifugal force is utilized to separate the heavy phase from the lighter phase by forcing the heavy phase to flow outwardly away from the rotational axis of the rotor and displace the lighter phase closer to the rotational axis of the rotor. These process streams are then individually collected at the upper end of the rotor at a location adjacent to the outer periphery thereof for the heavier liquid phase and at a location adjacent to the rotational axis of the rotor for the lighter liquid phase. During this introduction of the process liquids into the mixing zone, the two phases are introduced tangentally into a mixing zone to enhance mixing and mass transfer between the liquid phases so that a good mixture of the liquid phases enter the rotor through an opening in the base thereof due to pressure gradients developed by the rotation of the rotor. In nuclear reactor fuel reprocessing systems, the centrifugal contactors, which may employ the improvement provided by the present invention, are disposed in a multistage or cascade arrangement with each contactor having a diameter of approximately 5.5 centimeters. A detailed description of the use of centrifugal contactors in a multistage nuclear reactor fuel reprocessing operation is set forth in a report entitled Developments in Centrifugal Contactor Technology, R. T. Jubin et al, ORNL/TM-10768, U.S. Government Printing Office, September, 1988. This report is incorporated herein by reference.
While the present invention is particularly useful in centrifugal contactors such as those envisioned for the use of reprocessing nuclear fuels, it is to be understood that the present invention may be utilized in centrifugal contactors employed for the separation of other liquids such as those generally mentioned above.
It has been found that while the use of centrifugal contactors in a multistage system or cascade are desirable for continuous counter-current liquid extraction processes problems occur during the processing of liquids in the 5.5-cm-diameter contactors which are deleterious to the operation of the cascade. More specifically, if a centrifugal contactor at any location or stage within the cascade becomes inoperative such as caused by the failure of a drive motor, the entire cascade must be shut down. If the operation of the cascade was not halted the flow of process liquids, such as the aqueous and organic phases in a nuclear reactor fuel reprocessing operation, continues from adjacent stages into the inoperative stage until the rising liquid in the inoperative stage reaches the organic collector ring. At this time, the liquid in the inoperative stage will flow back flow into the process-liquid feed lines from the adjacent stages. This back flow affects the feed of the organic phase from the adjacent stage as well as the organic collector ring in the adjacent stage from which the organic stream is normally delivered to the inoperative stage since the organic phase collector rings are at the same elevation. Further problems occur in that the aqueous-organic liquid interface rises above the feed points in the inoperative stage so that the organic phase will flow as intended to the adjacent stage while the aqueous phase which is the heavier phase will tend to flow up the organic feed line. The interface formed between the aqueous and organic liquids in the inoperative stage rises so that the organic phase feed line becomes blocked with the aqueous phase liquid so as to interfere with the discharge of the organic liquid from the operating adjacent stage so as to eventually cause the flooding of all stages down stream of the inoperative stage.
Efforts to overcome this significant problem due to the deleterious contamination of the process liquids when one of the centrifugal contactors in a multistage operation became inoperative and which would also allow for continued operation with one inoperative stage have been successful, but only when the inoperative stage is not an end stage. The term "end stage" as used herein is either the stage at one end of the cascade from which the aqueous liquid phase is discharged for further processing or the stage of the cascade system from which the organic phase liquid is discharged for subsequent. It was previously found that this problem with respect to an internally located stage becoming inoperative could be overcome by installing horizontally oriented overflow ports below the organic collector rings and above the feed points with these overflow ports serially coupling the centrifugal contactors in the cascade. The overflow ports provide for the overflow of both aqueous and organic phases from the inoperative stage into the mixing volume in the operating adjacent stages at a location below the collector rings so as to allow for essentially complete flushing of the organic from the system since the interface of the aqueous and organic phases would occur at an evaluation in the mixing volume at a level common with the overflow ports. The initial flow of liquid through these ports from an inoperative stage would be the organic phase since there would be an inorganic layer of finite thickness over the rising heavier aqueous phase. However, when the organic layer becomes displaced, the overflow of liquid through the overflow ports would consist of both aqueous and organic phases in essentially the same proportions as the feed stream introduced into the inoperative centrifugal contactor. Thus, with equal volumes of process liquid flowing to each adjacent stage with the phase relationship in each stage being equal to that entering the inoperative stage the operation of the cascade could continue without being shut down as heretofore required.
While the use of the overflow ports provided a satisfactory solution to the failure or inoperativeness of internal stages of a multistage centrifugal contactor arrangement or cascade, the overflow ports were not useful when the inoperative stage was an end stage in the cascade. It was found that when either of the end stages became inoperative, the entire cascade operation had to be terminated since both the aqueous and organic phases would be discharged from the inoperative end stage through the conduit normally utilized for either the aqueous product with one end stage or the organic product with the other end stage. This mixing of the liquid phases in the discharge lines causes deleterious contamination of the product stream. When either end stage failed, process liquids in the inoperative end stage would rise until they back flow into the adjacent centrifugal contactor through the overflow port. This back flow would allow for the operation of the internal centrifugal contactors in the normal manner but a significant quantity of the contaminated liquid would be discharged from the cascade through the inoperative end stage so as to render unwarranted any further operation of the cascade.